The invention relates to a semiconductor device having a semiconductor body comprising a surface-adjoining region of monocrystalline silicon, which is laterally surrounded at least in part by a sunken oxide layer, and a highly doped silicon layer disposed thereon, the silicon layer being laterally separated substantially entirely from the region by an oxide layer, the region comprising a doped first zone of the same conductivity type as the silicon layer adjoining at least the edge of the region and an adjacent second zone, the silicon layer adjoining the first zone on an edge portion of the surface of the region, the second zone being provided with an electrode.
The invention further relates to a method of manufacturing the device.
A semiconductor device of the kind described above is known from the publication of Washio et al: "A 48 ps ECL in a Self-Aligned Bipolar Technology", ISSCC '87, pag. 58-59.
This publication discloses a bipolar transistor, which is provided in a mesa-shaped semiconductor region of silicon. The mesa-shaped region is located above a highly doped buried layer and is laterally enclosed by a sunken oxide layer and a polycrystalline silicon layer disposed thereon, which adjoins a highly doped base contact zone on the upper surface of the mesa and is laterally separated from the mesa substantially entirely by an oxide layer.
This known transistor has very small dimensions due to the fact that successful attempts have been made to cause the polycrystalline silicon layer to adjoin in a self-aligned manner only a very narrow edge region of the mesa.
However, a disadvantage of this construction is that the connection between the polycrystalline base connection and the base zone is established solely via the base contact zone diffused from the polycrystalline silicon. Irregularities in this edge region, which are liable to occur due to the fact that the so-called "bird's beak" edge structure of the sunken oxide layer is not always the same, can give rise to either a poor base connection or too short a distance between the polycrystalline base connection and the emitter zone. Due to the fact that the base contact zone and the emitter zone, which are both highly doped, adjoin each other, the emitter-base breakdown voltage can be considerably reduced, while in given circumstances the emitter-base junction can even extend partly in polycrystalline material, which can adversely affect the transistor properties.
It should be noted that in this Application the term "polycrystalline silicon layer" is to be understood to mean any non-monocrystalline silicon layer, therefore also, for example, an amorphous silicone layer.